As an electrophotographic color laser printer, there is known a tandem color laser printer which includes four photosensitive drums corresponding to toner of four colors such as yellow, magenta, cyan and black, respectively.
In the tandem color laser printer, since toner images of those four colors are formed individually on the corresponding photosensitive drums substantially simultaneously, a color image can be formed at almost the same speed as that at which a monochrome image is formed in a monochrome laser printer.
An image forming apparatus which is a tandem color laser printer includes two motors, a pair of transmission gears provided for each motor and four drum gears provided for the transmission gears, respectively. In the image forming apparatus, the four drum gears are driven by the two motors through the transmission gears. Phases of the pair of transmission gears are shifted from each other by a predetermined angle and the drive of the motors is controlled so that the phases of the transmission gears are synchronized, and occurrence of color registration error can be reduced or prevented.
An image forming apparatus includes four drum drive gears corresponding to four photosensitive drums, respectively, three idle gears provided between the four drum drive gears, respectively so that each of the idle gears meshes with the pair of drum drive gears adjacent thereto, and a gear of a drive motor provided to mesh with one of the drum drive gears. According to this configuration, the driving force of the drive motor transmitted to the one of the drum drive gears is further transmitted to the three remaining drum drive gears via the idle gears and the drum drive gears, whereby the respective photosensitive drums are rotated.
In such a tandem color laser printer, toner images of four colors are overlapped sequentially. Therefore, in order to reduce or prevent color registration error occurring during the overlapping of the toner images, it is important that the toner images are overlapped with the same printing errors occurring in the respective colors. Accordingly, it is advantageous that the photosensitive drums corresponding to the four colors are driven under the same conditions.
In the image forming apparatus as described above firstly, the two motors are provided, and the drive of the two motors are controlled in order to synchronize the phases of the transmission gears. That is, the image forming apparatus needs the two motors and two sensors for controlling the drive of the two motors, which makes the configuration of the image forming apparatus complex. Additionally, this configuration causes increase in the production cost of the image forming apparatus.
In the image forming apparatus as described above secondly, since the number of gears provided between the motor and the photosensitive drums are different with each other, the production errors in the respective gears affect the photosensitive drums differently, and the photosensitive drums cannot be rotated in the synchronized manner, which causes a fear that color registration errors are generated.